Surface-treated steel sheet coated with chromate film for fuel tanks and method of fabricating the same

ABSTRACT

The present invention relates to a surface-treated steel sheet coated with a chromate layer having excellent corrosion resistance, fuel resistance and seam weldability, and a chromate solution used for preparing the surface-treated steel sheet. Specifically, the chromate solution of the present invention comprises (a) a main solution prepared by adding to a chrome solution having 0.4 to 0.8 of trivalent chrome composition and 7 to 50 g/l of dissolved chrome, on the basis of the chrome in the chrome solution, 50 to 80 w % of phosphate, 20 to 40 w % of fluoric acid, 300 to 2000 w % of colloidal silica, 5 to 15 w % of sulfuric acid, and 25 to 40 w % of sodium phosphate, and (b) a hardener solution containing added epoxy silane or glycidyl silane of 2 to 10 w % on the basis of total hardener solution, the pH of which is controlled between 2 and 3, wherein 10 to 60 w % of the hardener solution being added to the main solution.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to an unleaded surface-treated steel sheetfor fuel tanks of an automobile, more specifically, a surface-treatedsteel sheet coated with a chromate layer having excellent cosmeticcorrosion resistance and fuel corrosion resistance as well as seamprocessibility and a method of fabricating the same.

(b) Description of the Related Art

Generally, it is required that fuel tanks for storing fuel of anautomobile should have resistance to corrosion on its outer surface tobe exposed to the atmosphere (hereinafter, referred to as the “cosmeticcorrosion resistance”) as well as resistance to corrosion on its innersurface to contact fuel such as gasoline (hereinafter, referred to asthe “fuel corrosion resistance”).

A fuel tank is usually made by press-forming steel sheets intocup-shaped upper and lower tank bodies, and welding the bodies to eachother by resistance welding method such as spot welding or seam welding,or joining method such as soldering or brazing. In this respect, becausetwo parts are joined each other in a fuel tank, good weldability andgood processibility are required for the steel sheet to be applied foruse in forming fuel tanks.

A ternesheet that is a cold-rolled steel sheet plated with lead-tinalloy has widely been used for fuel tanks. However, the ternesheet hasbeen limited in its use because it contains lead that causesenvironmental pollution.

Korean Patent Application No. 97-703448 and Japanese Patent Laid OpenPublication No. Hei9-59783 disclose unleaded surface-treated steel sheetthat can replace ternesheet. The above inventions relate to coatingspecial chromate solution on a zinc-nickel (Zn—Ni) alloy plated steelsheet. However, the above technique requires a minute crack-format onprocess on the zinc-nickel alloy plated layer before treating chromate,thus the manufacturing process is complicated. Furthermore, in such astructure, chrome is liable to elute even in contact with minimum amountof moisture contained in fuel, and this results in deteriorated fuelcorrosion resistance.

To solve these problems, the present inventors provided chromatesolution for a surface-treated steel sheet that does not need to formminute cracks on the plated layer and can prevent elution of chrome inthe international patent publication No. WO 00/32843.

As an automobile company using surface-treated steel sheet for fuel tanktries to develop the appearance of an automobile variously, necessity ofseam processing increases when manufacturing fuel tanks. When fuel tanksare manufactured by seam processing, a coating layer of surface-treatedsteel sheet which is subjected to seam-process forms cracks and peelsoff as well.

In addition, recent automobile companies have a tendency to replaceelectroplated steel sheet with melted zinc plated steel sheet in orderto reduce cost and to increase cosmetic corrosion resistance of anautomobile. Melted zinc plated steel sheet, however, is not appropriateto use as a fuel tank of automobile because it has problems such ascracking or exfoliation at the coating during seam processing even if ithas good cosmetic corrosion resistance.

Thus, there is a need for developing chromate solution and a process fortreating the same that satisfy cosmetic corrosion resistance and fuelcorrosion resistance as well as seam processibility at the same time.

SUMMARY OF THE INVENTION

Therefore, the present invention is derived to solve the above problems.The present invention provides an unleaded surface-treated steel sheetcoated with chromate which prevents cracks of coating layer during seamprocessing thereby improving cosmetic corrosion resistance and fuelcorrosion resistance after processing.

It is another object of the present invention to provide chromatesolution used for manufacturing surface-treated steel sheet coated withchromate which can improve cosmetic corrosion resistance and fuelcorrosion resistance after processing.

It is another object of the present invention to provide chromatesolution that adheres into melted zinc plated steel sheet and canprevent cracks or exfoliation due to abrasion between press plane duringseam processing.

It is another object of the present invention to provide a process forfabricating unleaded surface-treated steel sheet coated with chromatelayer which has excellent cosmetic corrosion resistance and fuelcorrosion resistance after processing.

In order to achieve the above objects, the present invention provides aresin solution comprising: (a) a main solution prepared by adding 50 to80 w % of phosphoric acid, 20 to 40 w % of fluoric acid, 300 to 2000 w %of colloidal silica, 5 to 15 w % of sulfuric acid, and 25 to 40 w % ofsodium phosphate on the basis of chrome in a chrome solution having 0.4to 0.8 of trivalent chrome rate and 7 to 50 g/l of dissolved chrome; and(b) a hardener aqueous solution comprising one of epoxy silane orglycidyl silane of 2 to 10 w % on the basis of total hardener solution,the pH of which being controlled between 2 and 3, said hardener aqueoussolution of 10 to 60 w % being added to the main solution. Wherein, pHof said hardener aqueous solution is controlled by phosphoric acid.

In order to achieve the above objects, the present invention provides amethod of fabricating surface-treated steel sheet for a fuel tank of anautomobile comprising the steps of: plating zinc/zinc-based alloy onsurface of low carbon content steel; coating the chromate solution ofclaim 1 on steel sheet plated with zinc/zinc-based alloy so as to have30-150 mg/m² of chrome content of coating layer after drying; and bakingsaid steel sheet coated with chromate solution at 140-250° C. Wherein,said plating step comprises soaking zinc/zinc-based alloy into steelsheet in electroplated or melted state.

In order to achieve the above objects, the present invention provides anunleaded surface-treated steel sheet coated with chromate solution.

According to the present invention, it is possible to provide anunleaded surface-treated steel sheet coated with chromate solution whichcompactly adheres melted zinc plated layer or zinc/zinc-based alloyelectric plated layer so that it can reinforce adhesion between steelsheet and coating layer while seam processing, and improve cosmeticcorrosion resistance and fuel corrosion resistance even afterprocessing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An unleaded surface-treated steel sheet according to the presentinvention is prepared by electroplated zinc or zinc-nickel alloy, orplating melted zinc on cold-rolled steel sheet and coating chromatesolution on the above steel sheet plated with eletroplating or meltedzinc plating.

Low-carbon steel sheet having less than or equal to 0.03% of carboncontent is used as a cold-rolled steel sheet in the present invention.

Hereinafter, chromate solution used for chromate layer coating isexplained in detail.

Characteristics of chromate solution used in the present invention areas follows:

First, in order to maintain secure adhesion between coated chromatelayer and zinc or zinc alloy layer, content of hardener solution addedto the chromate solution is optimized. Thus, it can remarkably preventdestruction of chromate coating layer when processing surface-treatedsteel sheet coated with chromate layer. Accordingly, it is not necessaryto form minute cracks on the surface of plated layer as prior art sothat the process become simple and cost is reduced.

Second, excellent fuel corrosion resistance is obtained by drasticallyreducing elution of six-valent chrome while increasing hydrophobicity ofchromate coating layer.

Third, since structure of chromate layer is improved by adding additivesinto chromate solution, the surface-treated steel sheet according to thepresent invention can maintain excellent cosmetic corrosion resistanceand fuel corrosion resistance even after processing.

The chromate solution of the present invention comprises main solutionand hardener solution.

The main solution comprises phosphoric acid, fluoric acid, colloidalsilica, sulfuric acid and sodium phosphate as additives in addition tochrome aqueous solution, wherein sodium phosphate is added to improvestructure of coating layer. Also, hardener solution comprises epoxysilane or glycidyl silane aqueous solution.

Chrome aqueous solution as a main component in the main solution isprepared as follows.

First, anhydrous chromic acid of 70-200 g/l is added into distilledwater and is dissolved. Then, ethylene glycol is added to dissolvedchromate and six-valent chrome is reduced to tri-valent chrome so thatratio of tri-valent chrome with respect to total chrome(reduction ratio)is 0.4-0.8. When reduction ratio is less than 0.4, it cannot obtaineffect of cosmetic corrosion resistance by tri-valent chrome and contentof elution chrome increases as soluble six-valent chrome increases.Meanwhile, when reduction ratio is more than 0.8, solution becomes gelso that storability of solution decreases.

Thus, concentration of chrome is set to 7-50 g/l by adding distilledwater into chrome aqueous solution. The reason for limitingconcentration of chrome relates to content of chrome added to a steelsheet. A roll coating process is used when chromate is coated on surfaceof plated steel sheet in the present invention. When the roll coatingprocess is used, content of chrome adhering to steel sheet depends onrotation speed of a roll and pressure applied between rolls. Therefore,considering these characteristics, it needs to prepare solution whichforms optimal coating layer. When content of chrome in chrome aqueoussolution is less than 7 g/l, the amount of chrome adhering to steelsheet is deficient, whereas when more than 50 g/l, the appearance of thesurface is not good and the stability of the solution decreases.

In order to improve material property of chromate solution prepared asabove, phosphoric acid, fluoric acid, colloidal silica and sulfuric acidare added.

Content of phosphoric acid is preferably 50-80 wt % on the basis ofchrome component. If phosphoric acid is less than 50 wt %, it is notsufficient to have improved effect of solution property and surfaceproperty. If phosphoric acid is more than 80 wt %, reduction ratioincreases so that stability of solution decreases, and excess phosphatedecreases cosmetic corrosion resistance.

Fluoric acid is added to improve cosmetic corrosion resistance andevenness of coating layer. If fluoric acid is added less than 20 wt % onthe basis of chrome component, cosmetic corrosion resistance is notsufficient whereas if more than 40 wt %, sludge occurs in the solutionso that stability of solution decreases. Therefore, content of fluoricacid is preferably 20-40 wt %.

Silicone oxide(SiO₂) having pH 2-4 in the colloidal state as silica isadded preferably in 300-2000 wt % on the basis of chrome component.Silica forms crosslink-bond during baking so that it prevents formationof zinc oxide at matrix metal. In addition, since silica is hydrophobic,it can increase cosmetic corrosion resistance against moisture andimprove adhesion of coating layer. However, if silica is less than 300wt %, the above effects are not sufficient whereas if silica is morethan 2000 wt %, stability of solution and adhesion of coating decrease.

Sulfuric acid can improve surface quality of steel sheet by regulatingcolor of solution. Also, it can improve stability of solution byimproving flow of solution. If sulfuric acid is less than 5 wt % on thebasis of chrome component, effect of material property of surfacedecreases whereas if more than 15 wt %, stability and cosmetic corrosionresistance of solution deteriorates.

The main solution of the present invention is prepared by adding theabove element into chrome solution, further adding sodium phosphate asexplained following, and mixing these.

The reason for addition of sodium phosphate is to improve materialproperty of chromate coating layer. That is, sodium phosphate canprevent cracks of coating layer during seam processing so that itmaintains cosmetic corrosion resistance and fuel corrosion resistanceafter processing. If sodium phosphate is less than 25wt %, effect ofimproving coating structure decreases. On the contrary, if sodiumphosphate is more than 40 wt %, precipitation occurs in the solution sothat stability of solution decreases. Therefore, content of sodiumphosphate is preferably 25-40 wt %.

Chromate solution of the present invention is prepared by adding ahardener into the main solution prepared as above.

Hardener added to main solution facilitates cross link bond of chromateso that it forms chrome coating layer and simultaneously serves asmaintaining adhesion of chromate coating layer and matrix metal. Thus,it is important to select hardener considering kind of zinc or zincalloy plating which plated on steel sheet and the above effect. Thehardener of the present invention includes epoxy silane and glycidylsilane considering the above.

First, a method of preparation for hardener solution comprising epoxysilane is as follows.

A hardener solution comprising epoxy silane is prepared by adding 2-10wt % of epoxy silane into distilled water and adding small amount ofphosphoric acid to prevent gelation when mixing with main solution.Thus, pH of hardener solution is set to be similar to that of mainsolution, which is 2-3. There are many method to set pH. Among those, itis preferable to add phosphoric acid. If epoxy silane is added less than2 wt %, chrome cross-link reaction rarely occurs. If epoxy silane isadded more than 10 wt %, storability of solution decreases.

If hardener solution comprising epoxy silane is added less than 10 wt %on the basis of main solution, cross-link reaction does not occursufficiently. If hardener solution comprising epoxy silane is added morethan 50 wt %, stability of solution decreases. Thus, content of hardenersolution comprising epoxy silane is preferably 10-50 wt % on the basisof main solution.

Then, a method of preparation for hardener solution comprising glycidylsilane is as follows.

A hardener solution comprising glycidyl silane is prepared by adding2-10 wt % of glycidyl silane into distilled water and adding smallamount of phosphoric acid to prevent gelation when mixing with mainsolution. Thus, pH of hardener solution is set to be similar to that ofmain solution, which is 2-3. If glycidyl silane is added less than 2 wt%, content of hardener needed to form chromate cross-link bond increasesand total content of chrome decreases so that amount of chrome added tosteel sheet decreases during chrome coating. If glycidyl silane is addedmore than 10 wt %, storability of solution decreases so thatprecipitation occurs in the solution.

If hardener solution comprising glycidyl silane is added less than 20 wt% on the basis of main solution, cross-link reaction of chromate doesnot occur at an appropriate temperature and time. Thus, chromates becomemacromolecule and do not form coating layer. And then, while heating andcooling it, most of chrome elutes so that cosmetic corrosion resistancedramatically decreases. If hardener solution comprising glycidyl silaneis added more than 60 wt %, hardener component of chromate solution issubjected to occur chromate reduction so that stability of solutiondecreases. Therefore, it is preferable to add 20-60 wt % of hardenersolution comprising glycidyl silane on the basis of main solution.

Particularly, glycidyl silane added to hardener solution reinforcesadhesion of plated layer and chromate coating layer while treatingmelted zinc plated steel sheet with chromate. This is because glycidylsilane increases reactivity with melted zinc plated layer, whichprevents exfoliation of coating layer or elution of surface bypenetration of solvent.

Chromate solution prepared as above can be used as steel sheet for fueltank by coating chromate on zinc-based alloy electroplated steel sheetor melted zinc plated steel sheet. In addition, resin may be coated onthe upper side of the chromate coating layer for a fuel tank.

Since zinc plated steel sheet of the present invention includeszinc-based alloy plated steel sheet such as Zn or Zn—Ni, the presentinvention is not limited to zinc-based alloy exemplified in thespecification.

Method of formation of chromate coating layer includes a reactive type,an electrolyte type, and a coating type. A reactive type and anelectrolyte type, however, occur water pollution because they makesix-valent chrome elution easily. Thus, a coating type is preferable interms of cosmetic corrosion resistance and minimization of pollution.

Three-stage roll coater can be used for coating chromate solution onzinc/zinc-based alloy plated steel sheet. The amount of chromateattached to the steel sheet is regulated by each roll driving direction,rolling speed, and each roll adhesion pressure.

The amount of chromate is preferably 30-150 mg/m² on the basis of driedcoating content on the plated steel sheet. If the content is less than30 mg/m², cosmetic corrosion resistance is not sufficient whereas If thecontent is more than 150 mg/m², it is not economical and chrome elutesto deteriorate characteristics of coating layer.

Baking temperature of the steel sheet coated with chromate is preferably140-250° C. If baking temperature is less than 140° C., hardeningreaction of coating layer is not sufficient. On the contrary, if thebaking temperature is more than 250° C., minute cracks occur on thesurface so that cosmetic corrosion resistance decreases.

Now, preferred embodiments are suggested to help the apparentunderstanding of the present invention. The below embodiments areprovided for the sake of clear understanding only and the presentinvention is not limited thereto.

EMBODIMENT EXAMPLE 1

Examples of chromate solution are prepared according to the method ofthe present invention in the composition of table 1.

TABLE 1 Chrome concentration (reduction Phosphoric Sulfuric FluoricSodium Colloidal Epoxy ratio 0.5) acid acid acid phosphate silicahardener Ex. Nos. (g/l) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) Com.Ex. 1 24 60 — 25 — 800 30 Com. Ex. 2 60 6 25 — 800 30 Com. Ex. 3 — 6 25— 800 30 Com. Ex. 4 60 6 25 — — 30 Com. Ex. 5 60 6 — — 800 30 Com. Ex. 660 6 25 — 800 — Ex. 1 10 60 6 25 35 800 30 Ex. 2 80 11 30 40 1300 20 Ex.3 60 8 40 30 1000 50 Ex. 4 24 50 13 35 33 1500 40 Ex. 5 70 10 30 27 180030

Comparative Example 2 of table 1 is the same composition as the chromatesolution used in International patent publication No. WO 00/32843 filedby the present inventors. Epoxy silane as a hardener was aproduct(KBM303) made by Japanese Shinetsu Kagaku company.

Chromate solution prepared as above was coated on zinc or zinc-nickelalloy electroplated steel sheet by using a three-stage roll coater. Theplating content of zinc or zinc-nickel alloy plated on steel sheet wasshown in table 2. Chrome content of chromate layer coated on electricplated steel sheet was 50-150 mg/m² on the basis of dried coating afterdrying.

The plated steel sheet coated with chromate solution was baked at140-250° C.

Cosmetic corrosion resistance and fuel corrosion resistance of thesurface-treated steel sheet prepared by the above process were measuredas follows.

Cosmetic corrosion resistance was measured by using a Salt Spray Testaccording to the Japanese Industrial Standard Testing (JIS Z 2371). Atester used in the salt spray test sprinkle 5% NaCl solution to thesurface of specimen by 1 kg/m² of pressure and 1 ml/hour at 35° C.Cosmetic corrosion resistance was separately measured on flat plane ofsteel sheet and processed part. A specimen cut in a size of 75×150 mm asa flat plane was placed in the salt spray tester and measured. Then, aspecimen was punched into 95 mmφ and formed into a cup with diameter of50 mm and height of 25 mm as a processed part. Cosmetic corrosionresistance was measured in an outer part. After passing determined time,the specimen was taken out from tester, washed with distilled water anddried. According to the amount of occurrence of rust, the grades ofcosmetic corrosion resistance were classified.

Another method to measure cosmetic corrosion resistance is a CyclicCorrosion Test. The cyclic corrosion test comprises spraying NaClsolution for 4 hours, drying at 60° C. for 4 hours and moisturizing at95% humidity and 50° C. for 18 hours. The above cycle is one cycle/Iday. Cosmetic corrosion resistance was measured as follows and shown intable 2.

A flat plane facing to fuel was cut into 95 mmφ, and formed to a cupwith diameter of 50 mm and height of 25 mm. Fuel comprising regulargasoline 30 ml and 5% of sodium chloride (NaCl) aqueous solution wasfilled in the cup and fuel corrosion resistance was measured.

In order to simulate driving situation of automobile, rocking equipmentwas used such that the solution contained in the cup was in rockingmotion. The cups were allowed to stand for 3 months. Then, the cups weretaken out, washed by distilled water, and dried. According to the amountof occurrence of rust, the grades of fuel corrosion resistance wereclassified as the same way as the cosmetic corrosion resistance andshown in table 2.

TABLE 2 cosmetic corrosion resistance SST CCT fuel (720 hr) (30 cycles)corrosion flat flat resistance Ex. Nos. Content of chromate treatingplane processing plane processing 3 months Com. Ex. A. melted Pb—Snalloy plated steel sheet Δ ⊚ X X ◯ (105 g/m²) Com. Ex. B Zn—Ni alloyelectroplating (20 g/m²) + ⊚ ⊚ ⊚ ◯ ⊚ Com. Ex. 2 (80 mg/m²) Ex. A-1 Zn—Nialloy electroplating (20 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 1 (80 mg/m²) Ex. A-2 PureZn electroplating (40 g/m²) + O ⊚ ◯ ⊚ ⊚ Ex. 1 (50 mg/m²) Ex. B Zn—Nialloy electroplating (20 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 2 (50 mg/m²) Ex. C Zn—Nialloy electroplating (20 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 3 (130 mg/m²) Ex. D Zn—Nialloy electroplating (20 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 4 (80 mg/m²) Ex. E Zn—Nialloy electroplating (20 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 5 (100 mg/m²) Ex. A-3Zn—Ni alloy electroplating (20 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 1 (130 mg/m²) Ex.A-4 Zn—Ni alloy electroplating (20 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 1 (100 mg/m²)Com. Ex. C Pure Zinc (40 g/m²) + special chromate X X X X Δ (30-50mg/m²) Com. Ex. D Zn—Ni alloy electroplating (20 g/m²) + special ◯ ⊚ ⊚ ◯X Com. Ex. E. Zn—Ni alloy electroplating (20 g/m²) X X X X X Com. Ex. FZinc plated steel sheet (20 g/m²) + X X X X X Com. Ex. 1 (10 mg/m²) Com.Ex. G Melted zinc plating (90 g/m²) + ⊚ X ⊚ X X Com. Ex. 2 (80 mg/m²) ⊚:excellent, ◯: good, Δ: fair, X: poor

As can be seen in table 2, cosmetic corrosion resistance and fuelcorrosion resistance of Examples A-E are superior to Comparison ExampleA, C-G. Particularly, cosmetic corrosion resistance after processing ofExample A to E coated with solution according to the present inventionis superior to that of coated with comparison Example 2. Specialchromate solution of table 2 used a solution comprising chrome aqueoussolution with concentration of chrome of 30 g/l and reduction ratio of0.8, phosphoric acid 10 wt %, cobalt 5%, and colloidal silica 50 wt %.

In addition, table 1 shows that Examples A-1 to A-4 coated with Example1 of the present invention were superior to Comparison Examples in termsof cosmetic corrosion resistance and fuel corrosion resistance. Also,Example A-2 coated on pure zinc electroplated steel sheet was superiorto Comparison Examples in terms of cosmetic corrosion resistance andfuel corrosion resistance.

Example 2

Examples of chromate solution were prepared according to the method ofthe present invention in the composition of table 3.

TABLE 3 Chrome Concentration (reduction Phosphoric Sulfuric FluoricSodium Colloidal hardener (wt %) ratio 0.5) acid acid acid phosphatesilica Epoxy Glycidyl Ex. Nos. (g/l) (wt %) (wt %) (wt %) (wt %) (wt %)silane silane Com. Ex. 7 24 60 6 25 — 800 30 Com. Ex. 8 60 — 25 — 800 30Com. Ex. 9 — 6 25 — 800 30 Com. Ex. 10 60 6 −5 — 800 30 Com. Ex. 11 60 6— — — 30 Com. Ex. 12 60 6 25 — 800 Ex. 6 60 6 25 35 800 30 Ex. 7 10 8011 30 40 1300 20 Ex. 8 24 60 8 40 30 1000 50 Ex. 9 50 13 35 33 1500 40Ex. 10 70 10 30 27 1800 30 Ex. 11 30 70 8 25 35 1000 40 Ex. 12 45 60 635 35 800 30

Comparison Example 7 of table 3 was the same as Comparison Example 2 oftable 1. Glycidyl silane as a hardener was a product(KBM403) made byJapanese Shinetsu Kagaku company. In addition, a process ofmanufacturing plated steel sheet used in the present invention was thesame as Example 1 except that melted zinc is plated on steel sheet.

Measurement of cosmetic corrosion resistance and fuel corrosionresistance was the same as Example 1 and table 4 showed the results.

TABLE 4 Cosmetic corrosion resistance SST CCT Fuel (720 hr) (30 cycles)corrosion Flat Flat resistance Ex. Nos. Content of chromate treatingplane processing plane processing 3 months Com. Ex. H melted Pb—Sn alloyplated steel Δ ⊚ x x  sheet (105 g/m²) Com. Ex. I-1 Melted Zn plating(90 g/m²) + ⊚ x ⊚ x x Com. Ex. 7 (80 mg/m²) Com. Ex. I-2 Melted Znplating (90 g/m²) + ⊚ x ⊚ x x Com. Ex. 8 (80 mg/m²) Com. Ex. I-3 MeltedZn plating (90 g/m²) + ⊚ x ⊚ x x Com. Ex. 9 (80 mg/m²) Com. Ex. I-4Melted Zn plating (90 g/m²) + ⊚ x ⊚ x x Com. Ex. 10 (80 mg/m²) Com. Ex.I-5 Melted Zn plating (90 g/m²) + ⊚ x ⊚ x x Com. Ex. 11 (80 mg/m²) Com.Ex. I-6 Melted Zn plating (90 g/m²) + ⊚ x ⊚ x x Com. Ex. 12 (80 mg/m²)Ex. F-1 Melted Zn plating (90 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 6 (80 mg/m²) Ex. GMelted Zn plating (90 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 7 (50 mg/m²) Ex. H Melted Znplating (90 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 8 (130 mg/m²) Ex. I Melted Zn plating(90 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 9 (80 mg/m²) Ex. J Melted Zn plating (90g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 10 (100 mg/m²) Ex. K Melted Zn plating (90 g/m²) +⊚ ⊚ ⊚ ⊚ ⊚ Ex. 11 (130 mg/m²) Ex. L Melted Zn plating (90 g/m²) + ⊚ ⊚ ⊚ ⊚⊚ Ex. 12 (100 mg/m²) Ex. F-2 Melted Zn plating (90 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex.7 (130 mg/m²) Ex. F-3 Melted Zn plating (90 g/m²) + ⊚ ⊚ ⊚ ⊚ ⊚ Ex. 7 (100mg/m²) Com. Ex. J Pure Zn (40 g/m²) + x x x x special chromate (30-50mg/m²) Com. Ex. K Zn—Ni alloy electric plating (20 g/m²) +  ⊚ ⊚  xspecial chromate (80 mg/m²) Com. Ex. L Zn—Ni alloy plating (20 g/m²) x xx x x Com. Ex. M Zn plated steel sheet (20 g/m²) + x x x x x Com. Ex. 7(10 mg/m²) Com. Ex. N Melted Zn plating (90 g/m²) + ⊚ x ⊚ x x Com. Ex. 7(80 mg/m²) ⊚: excellent, 0: good, Δ: fair, x: poor

As can be seen from table 4, Examples F to L were superior to ComparisonExamples H to N in terms of cosmetic corrosion resistance and fuelcorrosion resistance. Particularly, in Comparison Examples I-1 to I-6,epoxy silane was added as a hardener. As a result, adhesion betweenchromate layer and material was not sufficient so that exfoliationoccurred and cosmetic corrosion resistance decreased. Also, sincesolvent was penetrated, fuel corrosion resistance decreased. However,chromate solution prepared by using a hardener according to the presentinvention showed more excellent characteristics. Special chromatesolution of table 4 used a solution comprising chrome aqueous solutionwith concentration of chrome of 30 g/l and reduction ratio of 0.8,phosphoric acid 10 wt %, cobalt 5%, and colloidal silica 50 wt %.

If the Examples of the present invention compare with the ComparisonExamples, the Examples of the present invention were measured as showinggood cosmetic corrosion resistance because area of white rust occurrenceis relatively small in the salt spray test and cyclic corrosion test.This is believed that good cosmetic corrosion resistance of melted zincplated steel sheet is maintained even after seam processing due tochromate solution.

Particularly, melted zinc-plated steel sheet treated with chromatesolution of the present invention has improved cosmetic corrosionresistance and fuel corrosion resistance after processing because itprevents exfoliation of coating when fuel tank is subjected to seamprocessing.

What is claimed is:
 1. A chromate solution used for preparing asurface-treated steel sheet for a fuel tank of an automobile comprising(a) a main body solution and (b) a hardener aqueous solution in anamount ranging from 10 to 60% by weight of the main solution, whereinthe main solution comprises: a chrome aqueous solution where theconcentration of the chrome is in the range of 7 to 50 g/l, and a ratioof trivalent chrome to the chrome content is in the range of 0.4 to 0.8;phosphoric acid in an amount ranging from 50 to 80% by weight withrespect to the chrome content; fluoric acid in an amount ranging from 20to 40% by weight with respect to the chrome content; sulfuric acid in anamount of 5 to 15% by weight with respect to the chrome content;colloidal silica in an amount ranging from 300 to 2000% by weight withrespect to the chrome content; and sodium phosphate in an amount of 25to 40% by weight with respect to the chrome content; and the hardeneraqueous solution comprises epoxy-based silane or glycidyl silane in anamount ranging from 2 to 10% by weight with respect to the hardeneraqueous solution and has a pH of 2 to
 3. 2. The chromate solution for afuel tank of an automobile of claim 1, wherein the pH of said hardeneraqueous solution is controlled by phosphoric acid.
 3. The chromatesolution for a fuel tank of an automobile of claim 1, wherein the amountof hardener aqueous solution ranges from 10 to50% by weight of the mainsolution when epoxy-based silane is used.
 4. The chromate solution for afuel tank of an automobile of claim 1, wherein the amount of hardeneraqueous solution ranges from 20 to 60% by weight of the main solutionwhen glycidyl silane is used.
 5. A method of fabricating surface-treatedsteel sheet for a fuel tank of an automobile comprising the steps of:plating zinc/zinc-based alloy on surface of low-carbon content steel;coating the chromate solution of claim 1 on steel sheet plated withzinc/zinc-based alloy so as to have 30-150 mg/m² of chrome content ofcoating layer after drying; and baking said steel sheet coated withchromate solution at 140-250° C.
 6. The method of claim 5, wherein saidplating step is performed by electroplating the zinc/zinc-based alloy orplating melted zinc/zinc-based alloy on a steel sheet.
 7. Asurface-treated steel sheet prepared by the process of claim 5 or claim6.